Coming from Norway, but living in Germany for a long time, I completely agree that even dedicated fish mongers in Germany usually only have a small range of products. And even if you can always get some kind of fish, the selection of crustaceans and molluscs is often limited to one kind of shrimps and with some luck you might also get blue mussels.
I've never been to the fish market in Hamburg, but looking at pictures from there, it seems to me as if it is a large number of vendors, each however again with a quite limited selection. I would not take it for granted, that you there will find a broad selection of 'especially crabs, shrimps and mussels'.
When I want to buy fish and sea food, I almost always go to Metro. The selection varies a bit from store to store, but is usually very good by German standards. A great advantage is that you can always check online which products are in stock in each store. Here in Munich, I could now get four kinds of blue mussels, vongole, fasolare and scallops if I look in their mussels section. A great disadvantage is that they only do wholesale and do not sell to end-consumers. You will need a customer card issued to a company to shop there.
Another option is the deli chain Frischeparadies. They have around 10 stores in different German cities and usually a good selection of more top-level and pricey sea food.
There are of course other fish mongers in Germany with a larger than usual selection, but difficult to suggest one without knowing exactly where you are located. Here in Munich, there are a few at the Viktualienmarkt. In Berlin, I would perhaps suggest going to the department store KDW.